Atomic structure worksheet with handwritten notes and calculations for carbon-12.

A worksheet titled "Atomic Structure" with a table listing isotopes, atomic number, mass number, and other atomic properties. The first row for C-12 is filled in with handwritten red numbers: atomic number 6, mass number 12, 6 protons, 6 neutrons, and 6 electrons. A green arrow points to the mass number with the label "Mass #". Handwritten notes in red include "12-6 = # of n°" and "H".